Before you begin, please view the appropriate dielectric constant graph.

These graphs illustrate capacitance in picofarads per foot (pF/ft.), in various vessel diameters, generated by standard probes offered by Robertshaw. In the graphs, the sensing probe is calculated as being mounted in the center of a cylindrical vessel of the diameter shown. The diameters shown in these graphs are: 4in., 14 in., and 60 in. The probe capacitance values can be approximated for other vessel diameters by interpolation from the values obtained from these graphs/diameters.

To calculate picofarad values for any product level, the following values must be known:

1. Vessel diameter
2. Product dielectric value
3. Probe type and length
4. Desired ZERO & 100% points or the ZERO & DIFFERENTIAL points.

The procedure for utilizing the dielectric constant graphs for making calculations for a given installation is as follows:

Example: Assumed data

1. Vessel diameter is 14 in.
2. Product dielectric is 60.
3. Probe type is Model 740A-B072 (72 in. active length).
4. Desired ZERO point is 12 in. above the bottom of the active probe end and the 100% level point is 12 in. below the gland. Therefore, a 4 ft. level change occurs from ZERO to 100% (FULL).

CALCULATIONS

1. At a dielectric of 1 (air), the 740A-B graph shows the pF/ft. is approximately 4.5.
   
   
2. At a dielectric of 60, the graph shows the pF/ft. to be approx. 53.
   
   
3. With product level up to the ZERO level point (remainder of the probe is in the air).
   
   1. Immersed section = (1 ft. x 53)=53pF
   2. Remainder of probe (5 ft. x 4.5) = 22.5 pF
   3. Gland value = 24 pF
   4. Total ZERO value = 99.5 pF
      
      
4. With the product level at the 100% (FULL) point (Lower 5’ in product and the upper 1’ in air).
   1. Immersed section value (5 ft. x 53) = 265 pF
   2. Remainder of probe (1 ft. x 4.5) = 4.5 pF
   3. Gland value = 24 pF
   4. Total 100% value = 293.5 pF
      
      
5. The total CAPACITANCE CHANGE is 293.5 minus 99.5 = 194 pF. For quick calculation of CAPACITANCE CHANGE value only, subtract AIR pF/ft. (example 4.5) from PRODUCT pF/ft. (example 53) and multiply by the level difference in feet.
   Example: 53 minus 4.5 = 48.5. 48.5 x 4 = 194 pF.

For calculation of probes near an “infinite” flat wall or for probes mounted in a large cylindrical vessel where the probe is near the wall of the vessel, calculate the value from the appropriate probe graph using the graph vessel diameter as twice the distance the probe is from the flat wall or vessel wall. Take this value and multiply by .77 (77%). This is not exact, but is close enough for all probe picofarad calculations.

GENERAL NOTES:

1. Conductive Coating Applications:
   

CAUTION - conductive coatings on the probe will cause an error in measurement. ALWAYS use anti-coating transmitters for continuous measurement and shortstop probes/instruments for point level measurement where products will leave a conductive coating (see “special features” in selection guide matrix). For continuous measurement with anti-coating transmitters, always use a probe or probe mounting that provides the highest pF/ft. gain but does NOT EXCEED the span of the instrument. Gain can be increased by using our high gain 741A probe by mounting the probe close to the wall of the tank or vessel.

2. Dielectric/conductivity changes in product:
   

For continuous measurement of products that change in dielectric/conductivity, DO NOT USE any of our capacitance type instruments. For point level only (no adjustable differential), an ON/OFF instrument with a horizontally mounted probe or Proximity Plate can be used, provided the instrument is calibrated using the lowest dielectric/conductivity the process will ever have.

3. Where a tank or vessel is non-metallic or lined, a metal process ground must be provided. This can be accomplished by installing a ground rod, using a concentric shield or ground wire wrapped probe (free flowing, non-viscous products only), or installing another bare probe to serve as the ground rod. The least preferred choice is the ground wire wrap. The ground rod should be installed in close proximity and parallel to the probe in low dielectric/non-conductive products. For high dielectric/conductive products, it can be installed anywhere, as long as it is not above the tip of the measuring probe.
   
   
4. When installing a probe into a vessel through a pipe or nozzle, always sheath the portion of the probe going through the pipe/nozzle (unless a shortstop probe is used). If a shortstop probe is used (ON/OFF point level only), the shortstop length (not overall length) must extend a minimum of 4 to 6 in. (preferably 6 in.) out of the pipe/nozzle and into the tank or vessel (see P.S. Sheet for 727A/732A for required length.)
   

NOTE: Only Models 318A and 5100 with shortstop option can be used with 727A/732A probes.

5. Concentric Shield Option of Probes:

This option can be used for increasing the probe gain in low dielectric materials, linearizing the LEVEL (not volume) signal in a horizontal or non-symmetrical tank, or providing a process ground in a non-metal tank.

CAUTION: concentric shields can only be used in free flowing, non-viscous liquids. Concentric shields are always the same length as the probe.

6. Flumes, Weirs, and non-cylindrical or horizontal tanks:

Model 5000A with the “Smart Chip” option or Model 7000 or 7000A can be used for measurement of flow in flumes and weirs, or volume in non-cylindrical or horizontal tanks. Because the Model 5000A is programmed at the factory, flume/weir size/type/minimum-maximum flow must be submitted at order time. For tanks, a strapping table is preferred; otherwise, submit complete dimensions of tank including type ends/dimensions. The Model 7000 and 7000A are field programmable.